Scientific Method —

Some dinosaurs were poised for flight

A new study suggests that the dinosaur lineage that gave rise to birds had a …

One of the oddities of vertebrate genomes is that animals that fly, such as birds and bats, have smaller genomes than their closest relatives. Birds, in fact, have the smallest genome of any tetrapod outside of some amphibians, with relatively little repetitive or non-coding DNA. This feature appears to be adaptational, too: flightless birds have larger genomes than their flying relatives. The best guess is that flight is metabolically demanding, and cutting down on the amount of genome that needs to be copied leaves more energy free for taking to the skies.

But this raises an difficult question: which came first? Will a flying lifestyle create the pressure that produces a streamlined genome, or can only those animals that by chance have small genomes adapt to flying? Given that the flightless ancestors of modern species are extinct, it's a tough one to answer. But a report that will appear in the next edition of Nature may give us a clue.

The researchers based their study on another biological oddity: the size of a cell is generally proportional to the amount of DNA it contains. So bigger genomes tend to yield larger cells. The research team looked at the size of a specific type of cell in the bone in 26 existing tetrapods and checked whether cell size and genome size correlated. The accuracy or the correlation turned out to be roughly 60 percent for any given species. They then went back and applied the same technique to sections of fossilized dinosaur bones from over 30 species.

A clear pattern emerged: a single type of dinosaur, the therapods, had genome sizes comparable to birds. This was undoubtedly not a shock to the researchers, given that therapods have been identified as the evolutionary ancestors of birds. The results suggest that, at least among the dinosaurs, the therapods were the most likely to take flight, as they had a genome that was well-prepared for the metabolic stress.

There are a number of assumptions that went into this study, but it seems worth noting that in both of the two graphical representations of the data, I was able to identify the two different bat species chosen for analysis without knowing anything about the species name. To me, at least, this suggests the analysis has hit on a real effect.